What was found
FOR FLIGHT 4196 THE CREW REPORTED THEY NOTICED THE EXTERNAL POWER CONNECTED AND ASKED TO HAVE IT DISCONNECTED. ONCE GROUND PERSONNEL DISCONNECTED, CABIN CEILING LIGHTS TURNED OFF FOLLOWED BY SLIGHT ELECTRICAL BURNING SMELL THE AIRCRAFT WAS REMOVED FROM SERVICE. ALL STATED ELECTRICAL PROBLEMS CAUSED BY USE OF EXTERNAL POWER WHILE MEL 24-9 IN EFFECT. RESET POWER, ALL NORMAL WITH APU POWER APPLIED. THE AIRCRAFT WAS RETURNED TO SERVICE.